Perhaps "crumbling" is the wrong word, but my G3 has stress cracks emanating out from every one of the metal fasteners that hold the handles on (I actually won't carry it by the handles anymore lest one crack right off) and I also remember a pretty good hairline crack near the Apple logo on one side. (It's a huge pain for me to drag the thing out of the computer desk its hidden inside so I didn't feel up to verifying it.) I know that broken-off handles are a fairly common consequence of shipping the old towers, but mine was never shipped by mail (I got it from the original owner) and while it may of had a few tiny cracks when I got it I *know* they've been spreading without me manhandling it.
It's ironic that I recall Apple making a big deal about how their clear plastic cases were made of "tough polycarbonate plastic". While I'll grant the plastic is "harder" than the opaque plastic used in most normal computer cases it is definitely more brittle (almost like glass) so if it takes a hard enough impact it cracks instead of denting. My personal theory about spreading cracks in things like handles is that the complex curvy shapes Apple casts this brittle substance in by design build some tension into the resulting case parts, which means if the part doesn't fit *perfectly* or a minor impact deforms a mating part of the chassis then any crack once started will tend to propagate on its own. A more exotic theory I have is that it's *possible* some chemical, thermal, or photodegradation process is *naturally* forming tiny cracks in the material, which then propagate due to the built-in stress inherent in the curvy parts.
Anyway. Your photos are not really clear enough for me to see what's going on. If you think the iMac took a good smack that caused the original crack, *and* you can see that there are two parts that are clearly out of register, *and* you think that disassembling the machine and reassembling it "properly" would relieve some stress, then it might be worth doing. Otherwise... I'd put the blinders on and ignore it.